Rebellious sweetheart returning by train, you suspect (13)
I believe the answer is:
revolutionary
'rebellious' is the definition.
(thesaurus)
'sweetheart returning by train you suspect' is the wordplay.
'sweetheart' becomes 'lover' (I've seen this before).
'returning' says the letters should be written in reverse.
'by' is a charade indicator (letters next to each other).
'suspect' indicates an anagram.
'lover' reversed gives 'revol'.
'train'+'you'='trainyou'
'trainyou' anagrammed gives 'utionary'.
'revol'+'utionary'='REVOLUTIONARY'
(Other definitions for revolutionary that I've seen before include "New" , "agitator" , "Causing dramatic change" , "Innovative" , "Insurgent - involving dramatic change" .)